This invention relates to a dry etching method employed in such applications as production of semiconductor devices, and more particularly to a dry etching method for etching a polycide film whereby high selectivity, high anisotropy and low pollution can be achieved without using a chlorofluorocarbon (CFC) gas.
A polycide film formed by stacking a polysilicon layer and a refractory metal silicide layer composed of a tungsten silicide or WSi.sub.x, for example, has been used broadly for recent years as a gate metallization material for LSI, because the polycide film exhibits resistance smaller than a single-layer film of a polysilicon by one order of magnitude for an identical cross-sectional area.
Since anisotropy should be realized for two different materials, the polycide film has brought about a new type of difficulty to the dry etching technique. That is, defects such as undercuts and constrictions tend to be generated on a pattern, because the lower layer or the polysilicon layer is etched faster than the upper layer or the refractory metal silicide layer due to the difference in vapor pressure of a resultant halogen compound, and because a reaction layer is formed on the boundary surface between the polysilicon layer and the refractory metal silicide layer. These etching defects cause generation of an offset region where impurities are not introduced at the time of ion implantation for generating source; and drain regions, and a reduction in dimensional precision at the time of forming sidewalls for realizing an LDD structure, and therefore, are unallowable particularly in a submicron device. Accordingly, a lot of research for realization of anisotropic processing for the polycide film is under way.
Conventionally, a chlorofluorocarbon or CFC gas typified by CFC-113 (C.sub.2 Cl.sub.3 F.sub.3) has been used broadly as an etching gas for the polycide film for the following reasons. F and Cl atoms in each molecule promote both a radical reaction and an ion assisted reaction, and deposited carbonaceous polymers perform sidewall protection, thereby making it possible to carry out anisotropic etching with a high etchrate.
However, since the CFC gas is pointed out as a cause of destruction of the earth's ozone layer, as commonly known, there is a pressing need to find appropriate alternative substances to the CFC gas and effective applications thereof.
As a CFC-free measure, the present applicant proposed, in the Japanese patent gazette, KOKAI TOKKYO KOHO No. 3-215938, a two-stage etching technique for etching a WSi.sub.x layer on the upper side of a tungsten polycide film, referred to as a W-polycide film hereinafter, by using HBr/SF.sub.6 mixed gas, while etching a polysilicon layer on the lower side by using HBr gas alone. This technique has significant advantages such as capability of achieving a practical etchrate, high anisotropy and high selectivity for a gate oxide film while preventing particle pollution due to WBr.sub.x or tungsten bromide.
Another CFC-free measure is a method of etching a W-polycide film by using Cl.sub.2 /CH.sub.2 F.sub.2 mixed gas instead of CFC-113, which is reported in the Extended Abstract of the 52nd Fall Meeting of the Japan Society of Applied Physics and Related Societies, 1991, p. 508, 9a-ZF-6. With this gas system, sidewall protection is performed by depositing carbonaceous polymers formed in the gaseous phase from CH.sub.2 F.sub.2. Also, if the flow rate of CH.sub.2 F.sub.2 is optimized, selectivity between the WSi.sub.x layer and the polysilicon layer can be increased, and residues on steps can be reduced.
Meanwhile, a method for achieving high anisotropy by reducing the temperature of a substrate to be etched (wafer) instead of providing sidewall protection effects by using the carbonaceous polymer has been proposed. This method, so-called low-temperature etching, is designed to keep the wafer at a temperature below 0.degree. C., thereby maintaining an etchrate in the direction of depth at a practical level with the assistance of ions while freezing or inhibiting a radical reaction on the sidewalls of patterns and preventing such etching defects as undercuts. A typical example of this method is reported in the Extended Abstract of the 35th Spring Meeting of the Japan Society of Applied Physics and Related Societies, 1988, p. 495, 28a-G-2, wherein a silicon trench and an n.sup.+ type polysilicon layer are etched by using an SF.sub.6 gas with a wafer cooled to -130.degree. C.
Although CFC-free measures have been thus proposed, problems remain to be solved for each of these measures.
For instance, practicality of the two-stage etching technique by using a HBr/SF.sub.2 mixed gas and a single HBr gas depends upon how precisely the timing for a change of gas composition between the WSi.sub.x layer and the polysilicon layer can be decided. If the timing for the change is too early, the remaining WSi.sub.x layer reacts with the single-composition HBr gas thereby forming WBr.sub.x of low vapor pressure, and therefore, deterioration of the particle level is very likely to take place. On the contrary, if the timing for the change is delayed, undercuts will be generated on the polysilicon layer by F.sup.*.
Besides, these days there are cases in which the polycide film must be etched on a substrate having large steps, as seen in bit line processing for SRAM or a transfer electrode forming process for CCD. In these cases, the decision on the timing is more difficult.
On the other hand, the method using the Cl.sub.2 /CH.sub.2 F.sub.2 mixed gas has a problem of excessive deposition of CH.sub.2 F.sub.2. It is reported in the Extended Abstract of the Dry Process Symposium, 1988, p. 74, II-8, that a CH.sub.2 F.sub.2 gas forms relatively rigid polymers compared with C.sub.4 F.sub.8, C.sub.2 Cl.sub.2 F.sub.4 (CFC-114), and CCl.sub.4 gases, has a low etchrate by incident ions. Accordingly, the use of CH.sub.2 F.sub.2 is likely to deteriorate reproducibility and the particle level.
In addition, carbon existing in the etching reaction system deteriorates selectivity for SiO.sub.2 based material layers. This problem is pointed out, for instance, in the Monthly Journal "Semiconductor World," published by the Press Journal Inc., January 1990, p. 81-84. In this case, when the carbon is adsorbed on the surface of an SiO.sub.2 based material layer like a gate oxide film, a C--O bond with a high bond energy (257 kcal/mole) is formed, thereby weakening an Si--O bond or reducing SiO.sub.2 to Si and making it more likely to be extracted by a halogenous etchant. This problem becomes particularly serious in performing a gate processing on a thin gate oxide film.
On the other hand, a low-temperature etching is expected to be one of promising CFC-free etching methods. However, if this method attempts to achieve high anisotropy only by freezing or inhibiting a radical reaction, a substrate to be etched (wafer) needs to be cooled to such a degree as to require liquid nitrogen. Hence, hardware related problems arise, such as increased demand for large-scale special cooling equipment and decreased reliability of vacuum sealant. Also anticipated is that additional time taken to cool the wafer below a room temperature and heat it back to the room temperature leads to a reduction in throughput as well as economy and productivity.